media: coda: do not enforce 512-byte initial bitstream payload on CODA960

On CODA960, sequence initialization can succeed if less than 512 bytes
are ready in the bitstream ring buffer.
On other variants, warn about too small payload in start_streaming.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>